Conditional query plans.

Hi.

This whole message might be a giant brain fart but I had an 
interesting idea today.

I was confronted by an obscene query plan. I have a table of logins 
that shows when webmail accounts were created. So a spammer went and 
set up 20 or so spam accounts. So I got a list by his IP and the time 
when he set them up. Now to batch cancel them I hacked up a quick 
query:
update users set enabled='f',disablereason='We do not allow our 
system to be used for SPAM.' where id in (select id from users where 
loginid in (select distinct loginid from logins where 
ip='123.123.12.12'));

This is a horrible way to do it and the query plan is even worse:
NOTICE:  QUERY PLAN:

Seq Scan on users  (cost=0.00..612996782699.54 rows=18180 width=172)SubPlan ->  Materialize
(cost=33718194.83..33718194.83rows=18180 width=4)  ->  Seq Scan on users  (cost=0.00..33718194.83 rows=18180 width=4)
SubPlan     ->  Materialize  (cost=1854.65..1854.65 rows=48 width=12)      ->  Unique  (cost=1853.44..1854.65 rows=48
width=12)      ->  Sort  (cost=1853.44..1853.44 rows=482 width=12)        ->  Index Scan using logins_ip_idx on logins

(cost=0.00..1831.97 rows=482 width=12)

Given that the first and second subplan actually return only 25 rows, 
there are 2 possibly distillations of this plan:

update users set enabled='f',disablereason='We do not allow our 
system to be used for SPAM.' where id in 
(27082,27083,27084,27085,27086,27087,27088,27089,27090,27091,27092,270
97,27098,27099,27101,27102,27103,27104,27094,27096,27095,27106,27100,2
7105,27093);

Which comes up with a plan:
NOTICE:  QUERY PLAN:

Index Scan using users_pkey, users_pkey, users_pkey, users_pkey, 
users_pkey, users_pkey, users_pkey, users_pkey, users_pkey, 
users_pkey, users_pkey, users_pkey, users_pkey, users_pkey, 
users_pkey, users_pkey, users_pkey, users_pkey, users_pkey, 
users_pkey, users_pkey, users_pkey, users_pkey, users_pkey, 
users_pkey on users  (cost=0.00..57.04 rows=2 width=172)

Basically it's going through each of the 25 as though they were 
separate updates.

The second and probably less optimal plan would be to create a hash 
of these 25 answers and do a sequential scan on users updating rows 
where id is found in that hash.


For these 2 query plans, 1 would be optimal in the event there is a 
small list to update, and the other would be ideal in the event there 
is a large list to update. 

Why attempt to formulate a complete query plan at the outset. Could 
you not break the query into smaller parts and re-optimize after 
every subplan completes? This way you would have an exact number of 
rows provided from the subplans so more accurate choices could be 
made farther down the line? This becomes especially relevant on large 
joins and other complex queries.

Maybe I just gave away an idea I could have sold to Oracle for 
millions, and maybe everyone is already doing this. Anyway, it's just 
thoughts and if anyone makes it this far it might be worthwhile for a 
little discussion.

-Michael
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